1. Field of the Invention
The invention relates to a low noise electron gun for use in electron tubes such as storage tubes, camera tubes, display tubes and the like. The invention is particularly suited for use in beam deflection tubes in which the beam generated by the gun is scanned across a target responsive to electrons by a deflection coil system or the like to either display on or read information stored by the target.
2. Description of the Prior Art
A tube of this type is described, for example, in an article entitled "An Experimental Light-Weight Colour Television Camera" in Vol. 29, Philips Technical Review, No. 11, 1968, pages 325-335. The electron beam in the camera tube described in the article is generated by a triode gun having a cathode, a control grid which is at a negative potential with respect to the cathode and an anode which is at a positive potential with respect to the cathode. The control grid and anode form a lens which focusses the electrons emitted from the cathode to a spot or "cross-over" in the region of the anode. The cross-over is then imaged on a photoconductive target by an electron lens and scanned across the target by a deflection coil system.
One important factor which affects both the resolution and the response rate of such camera tubes is the velocity distribution of the electrons in the beam. The velocity distribution is dependent on the temperature of the cathode and theoretically the best that can be obtained is a Maxwellian distribution corresponding to the actual cathode temperature. In practice, however, the velocity spread of the electrons is greater than that which would correspond to the Maxwellian distribution for the actual cathode temperature. One reason for the increased velocity spread is the interaction between electrons in the beam, particularly at the beam crossover, since electrons moving along intersecting tracks will repel each other causing one to move faster and the other one to slow down. In addition, x-ray radiation emitted by electrons impinging on the anode and positive ions striking the cathode may also release fast electrons which increase the velocity or energy spread of the electron beam.
The beam velocity distribution imposes a lower limit on the diameter of the spot to which the scanned beam can be focussed on the target and hence the resolution of the camera tube. As stated earlier, the response rate, that is the speed with which the tube reacts to variations in the intensity of the incident light, is also affected by the electron velocity distribution. Ideally all elements of the target should be stabilized at the same potential after scanning. However, as the velocity distribution of the electrons in the beam increases, the electrons with excessively high energies will cause the target to be charged to a lower potential than that desired increasing the beam-discharge lag and adversely affecting the response rate of the tube.
For the above reasons, it has been proposed to reduce the electron velocity distribution in the beam by using an electron gun which does not have a beam crossover. U.S. Pat. Nos. 3,894,261 and 3,226,595, for example, disclose electron guns of this type comprising a cathode and an anode which is operated at low positive potential with respect to the cathode. U.S. Pat. No. 3,831,058 discloses another gun of this type having a cathode, an apertured control grid which is operated at a negative voltage relative to the cathode and an apertured anode which is preferably 50 volts, and at most 125 volts, positive relative to the cathode. Because of the low positive voltage on the anode, the lens formed by the electrodes has a very large focal distance relative to the dimensions of the electron gun so that there is no crossover of the beam in the region between the cathode and the anode. Although such arrangements, by eliminating the crossover, reduce interactions between the electrons and hence reduce the energy spread in the beam, they, nevertheless, have several disadvantages.
One such drawback is that a "no-crossover" gun, when used in a camera tube with a magnetic focussing lens, produces an interference signal due to the effects of the return beam. The term "return beam" as used herein refers to that portion of the primary electron beam incident on the target which returns from the target back toward the electron gun end of the tube. The return beam is comprised primarily of electrons reflected from the target and the electrons in the primary beam which are not accepted by the target, because, particularly at low incident light intensities, portions of the scanned target are at nearly the same or even slightly negative potential with respect to the cathode. As the electrons in the return beam travel back toward the gun end of the tube, they are focussed by the magnetic lens onto the anode of the electron gun and scanned across it by the deflection coil system resulting in an emission of secondary electrons. The secondary electrons and the electrons in the return beam which are reflected from the anode have energies corresponding to the anode potential, which in an electron gun without a crossover is close to the cathode potential. Since the energy levels of these electrons are comparable to the energy of the electrons in the primary beam, the secondary and reflected electrons will once again be focussed on and scanned across the target producing an interference signal which appears as a "dark spot" in the visual image.
Another significant problem with guns of this type is that during operation, it is often necessary to vary the beam current by a factor of 5 to 10. In electron guns without a crossover, the beam current is directly proportional to the cathode current and therefore any increase in the beam current also increases the cathode current by the same factor resulting in a heavy load on the cathode which results in a sharply reduced cathode lifetime. Furthermore, in systems wherein dynamic beam control is effected by feedback coupling of the video signal to the anode, control signals with large amplitudes are required to vary the beam current.